The invention relates to a focussing apparatus for a camera, and more particularly, to such apparatus in which light from an object being photographed which is illuminated from a focussing light source is received to determine an in-focus position of a taking lens automatically.
As is well known, there are a variety of arrangements in which light from a focussing light source is reflected by an object being photographed, with the reflected light being received to determine an in-focus position of a taking lens. However, in the conventional arrangements, the focussing light source is deactivated before a shutter opening takes place in order to prevent a photographic film surface from responding to the focussing light. To this end, there is provided a mechanical switch which is operated immediately before the shutter is opened. This adds to the complexity of the arrangement and suffers from a disadvantage that a poor contact or malfunctioning may occur.
It will be noted that a conventional focussing apparatus includes a battery checker circuit which is connected in shunt with an in-focus detector circuit in order to prevent a malfunctioning thereof when the voltage of a power supply is reduced. In this manner, the operation of the in-focus detector circuit is prevented whenever a battery, serving as a power supply, is exhausted. However, the prior art employed an arrangement in which the focussing light source is illuminated independently from whether or not the in-focus detector circuit is in operation. As a result, the inoperative condition of the in-focus detector circuit, which is caused by the exhausted battery, is mistaken for a failure of the camera.
Before describing the invention, the described problems of the prior art will be considered in more detail in terms of a specific arrangement. FIG. 1 is a block diagram of an exemplary electrical circuit of a fundamental arrangement for a conventional focussing apparatus for a camera. The apparatus comprises a focussing light source B.sub.1 for producing focussing light L which is directed toward an object 1 being photographed, a light source control circuit LC which controls the energization or deenergization of the light source B.sub.1, an in-focus detector circuit AF responsive to the focussing light as reflected from the object 1, which is illuminated by the illumination from the light source B.sub.1, by determining the focussing condition, a battery checker circuit BC for disabling the in-focus detector circuit AF whenever an output voltage from a battery E.sub.1, operating as a power supply, has fallen below a given voltage, an electromagnet Mg.sub.1 responsive to an output from the in-focus detector circuit AF, a normally open main switch S.sub.1 which is closed before the initiation of operation of a shutter (not shown) in response to a first stroke of depression of a shutter release button (not shown), and a normally closed deenergization switch S.sub.2 which is turned off immediately before the shutter opening.
Specifically, a positive bus L.sub.2 is connected to the positive terminal of the battery E.sub.1, the negative terminal of which is connected through the main switch S.sub.1 to a negative bus L.sub.1. The circuits LC, AF and BC are connected across the buses L.sub.1, L.sub.2 so as to be fed from the battery whenever the switch S.sub.1 is closed. The battery checker circuit BC has its output fed to a control input of the in-focus detector circuit AF, which in turn has its output connected to the bus L.sub.2 through the electromagnet Mg.sub.1. The switch S.sub.2 is connected between the bus L.sub.1 and a control input of the light source control circuit LC.
FIG. 2 is a circuit diagram more specifically illustrating the light source control circuit LC of the focussing apparatus shown in FIG. 1. Specifically, the control circuit LC comprises a light source deenergizing transistor Q.sub.A, light source controlling transistors Q.sub.1 to Q.sub.3, time constant capacitor C.sub.1 and resistor R.sub.1, discharge diode D.sub.1 and resistors R.sub.2 to R.sub.5. The transistor Q.sub.A is formed by an NPN transistor having its base connected through resistor R.sub.5 to the bus L.sub.2 and also connected through the switch S.sub.2 to the bus L.sub.1. The emitter of the transistor Q.sub.A is directly connected to the bus L.sub.1 while its collector is connected to the bus L.sub.1 through the reversely poled diode D.sub.1 and also connected to the junction between the capacitor C.sub.1 and resistor R.sub.1. The capacitor C.sub.1 has its one end connected to the bus L.sub.2 and its other end connected to one end of resistor R.sub.1, the other end of which is connected through resistor R.sub.2 to the bus L.sub.1. The junction between the resistors R.sub.1 and R.sub.2 is connected to the base of the transistor Q.sub.1. The transistor Q.sub.1 is formed by an NPN transistor having its emitter directly connected to the bus L.sub.1 and its collector connected through resistor R.sub.3 to the bus L.sub.2 and also connected to the base of the next following transistor Q.sub.2. The transistor Q.sub.2 is again formed by an NPN transistor, and has its emitter connected directly to the bus L.sub.1, and its collector connected through resistor R.sub.4 to the bus L.sub.2 and also connected to the base of the final stage transistor Q.sub.3. The transistor Q.sub.3 is also formed by an NPN transistor, and has its emitter directly connected to the bus L.sub.1 and its collector connected through light source B.sub.1 to the bus L.sub.2.
In FIG. 2, reference character Q.sub.B represents an electromagnet controlling transistor having its collector-to-emitter path shunted by a Zener diode ZD.sub.1 which prevents the occurrence of spikes resulting from a back electromotive force of the electromagnet Mg.sub.1. The transistor Q.sub.B is formed by an NPN transistor, and has its base connected to the output of the in-focus detector circuit AF, its emitter directly connected to the bus L.sub.1 and its collector connected through the electromagnet Mg.sub.1 to the bus L.sub.2. It will be noted that the diode ZD.sub.1 is forwardly connected across the emitter and collector of the transistor Q.sub.B.
It is to be understood that the in-focus detector circuit AF of the present focussing apparatus includes an exposure control circuit so that an output from the detector circuit AF energizes the electromagnet Mg.sub.1 in order to control an exposure period, in addition to controlling the focussing operation. (Refer to the following description of the operation of the invention for detail.)
In operation, immediately before a shutter is to be opened, the switch S.sub.2 is turned off (opened) to permit the transistor Q.sub.A to be turned on. If then the capacitor C.sub.1 has not been completely charged and the light source B.sub.1 is illuminated, a rapid charging of the capacitor C.sub.1 is permitted through the collector-emitter path of the transistor Q.sub.A, causing the transistor Q.sub.1 to be turned off. This turns the transistor Q.sub.2 on and turns the transistor Q.sub.3 off, thus deenergizing the light source B.sub.1. Accordingly, a photographic film is prevented from being exposed to the focussing light from the light source B.sub.1. However, the use of mechanical switch S.sub.2 adds to the complexity of the arrangement and causes a failure due to poor contact or malfunctioning.
When an output voltage from the battery E.sub.1 has reduced below a given value, this fact is detected by the battery checker circuit BC, which in turn disables the in-focus detector circuit AF. Accordingly, no focussing operation takes place, and the taking lens is not displaced from its initial position. However, it will be noted that the battery checker circuit is not associated with the control circuit LC, and hence the light source B.sub.1 will be illuminated even when the output voltage from the battery E.sub.1 has reduced, in the same manner as when a normal voltage prevails. Thus it will be seen that the source B.sub.1 is illuminated even though no focussing operation takes place, and hence there is a likelihood of mistaking the absence of a focussing operation for a mechanical or electrical failure of the arrangement since the source B.sub.1 is illuminated.